Small Light Box

You might also try the classic approach used by microscopists, who are also severely limited in working distance...the ping pong ball. Cut one in half, drill or hack a hole in the center, and tape it to the front of your lens with double-stick or whatever. Then shine your lights onto the ping pong ball and it will give you wide and even diffusion. To test out how well it might work, cut a hole in an index card and tape to the lens in the same way. I've found the index card (or even better, vellum) often works better than the ping pong ball...Ray